1. Field of the Invention
This application is in the general field of analytical chemistry, and more specifically, it is a device and method of use in the field of analytical biochemistry and cell culture. This invention provides an innovative method for measuring the hydrogen ion concentration in aqueous solutions, or the negative logarithm of the hydrogen ion concentration (“pH”).
2. Background
Cell cultures are very sensitive to the pH of cell culture media, and monitoring and control of the media acidity are required for optimized cell growth. The object of the present invention is to provide a method for measuring pH of a solution without drawing a sample and without making contact with the solution thereby potentially contaminating it.
There are very few off-the-shelf available pH measurement systems that can provide fast, accurate, reliable, non-invasive measurement techniques and can be used for the automated monitoring and control of medium pH. The determination of pH currently requires that a sample be drawn from the solution, at which time one of three testing methods can be used. In one method, an electrochemical or optical pH probe is placed into the fluid, and a digital readout is obtained. This method requires a significant amount of solution to be drawn, and the testing process can contaminate the solution. Furthermore, highly accurate equipment can be expensive. The second method is less expensive and requires that a drop of solution be placed on filter paper impregnated with a colorimetric reagent so that a user can visually compare the color change to a reference chart. Although this method is inexpensive and requires little wasted sample, it is neither accurate nor precise, and it cannot be automated. In the third common method, a measured amount of color indicating solution is added to a measured amount of solution sample. This process is similar to the method using indicator paper in that its accuracy depends on the user's ability to discern color contrasts. The color indicating solution technique also requires that a significant amount of fluid be drawn for testing and then contaminated with the color change solution so that it is no longer useful for scientific purposes.
Thus, disadvantages of current pH test methods include the following: 1) a sample of the solution must be taken and exposed to a test device (pH probe, indicator paper, indicator chemicals, etc.), which potentially contaminates the sample and makes it unusable, resulting in wasted fluid; 2) color indicator paper and chemicals provide low accuracy and precision due to their reliance on the visual judgment of the user; 3) probe systems often exhibit short life span; 4) the probe system is the only technique that lends itself to automated controls; and 5) all of these methods are invasive.
The present invention provides a method for measuring pH of a solution without drawing a sample and without making contact with the solution (and thereby potentially contaminating it). The present invention thereby overcomes all of these disadvantages.
Although using the absorption of different wavelengths of light to determine pH is not a novel concept, the present invention comprises a method for data sampling and monitoring that is novel. Specifically, the present invention utilizes an absorbance ratio principle not used in prior art and an electronic means of automatic measurement and reporting.
The invention is especially useful in the field of cell culture, where culture media often contain pH indicator dyes. The invention is a useful replacement for immersible pH probes, which must interact directly with the liquid. It is an object of the present invention to avoid contact with cell culture liquids, which must be kept sterile and which can foul immersible devices that depend on chemical reactions.